Glucose-6-phosphate dehydrogenase: the key to sex-related xenobiotic toxicity in hepatocytes of European flounder ( Platichthys flesus L.)?

The role of glucose-6-phosphate dehydrogenase (G6PDH) in oxidative stress responses was investigated in isolated intact living hepatocytes of immature female and male European flounder ( Platichthys flesus L.) because it is the major provider of NADPH needed as reducing power for various detoxification pathways. Hepatocytes were exposed to sublethal concentrations of effective prooxidants such as 100 μM hydrogen peroxide (H 2O 2), 100, 200 and 400 μM benzo[a]pyrene (B[a]p) and 50 μM nitrofurantoin (NF) during culture. Since there is evidence that 17-β-estradiol inhibits certain pathways of xenobiotic biotransformation, we tested also the effects of different concentrations of 17-β-estradiol (0.2, 1 and 2 μM) alone and 1 μM in combination with 100 μM B[a]p on G6PDH activity. After short-term (1 day) and long-term (9 days) exposure, G6PDH activity was quantified in intact living hepatocytes by a tetrazolium salt method using tetranitroblue tetrazolium salt (TNBT). Hepatocytes obtained from male fish generally showed higher G6PDH activity than those of females. We observed significant inhibition of G6PDH activity by all oxidative stressors and 17-β-estradiol in both sexes of fish independently of culture conditions, but inhibition was stronger in cells of females than in cells of males. A cumulative effect of the steroid and B[a]p was not found. Our results indicate a sex-dependent inhibitory effect of all stressors and 17-β-estradiol on G6PDH activity in flounder hepatocytes independent of prooxidant activity of the specific compound. Consequently, NADPH supply for xenobiotic detoxification and other cellular antioxidative defence mechanisms may be different in livers of female and male flounder. The strongly decreased supply of NADPH in hepatocytes of females may explain the reduced and/or delayed NADPH-dependent activity of xenobiotic biotransformation systems such as cytochrome P450 (CYP450) and a lower capacity of non-enzymatic defence systems such as reduced glutathione that is particularly observed in female flounder. Moreover, the strong inhibition of G6PDH in livers of female flounder may explain higher susceptibility for xenobiotic toxicity and, therefore, potentially a higher risk to develop liver cancer.